THE STORY OF COULOMBIC CRITICALITY

Recent experiments on phase separation and criticality in ionic fluids are reviewed briefly. The data suggest a sharp distinction between so lvophobic criticality, displayed by nonionic fluids and some electroly tes, that is associated with Ising-like exponents, beta congruent-to 0 .325, gamma congruent-to 1.239, and nu congruent-to 0.631, and Coulomb ic (or ionic) criticality characterized by classical, van der Waals ex ponents, beta = 0.5, gamma = 1, and nu = 0.5. Only experiments on the sodium-ammonia system seem to straddle this dichotomy: they show cross over from classical to Ising behavior close to T(c) al a characteristi c crossover scale t(x) = \T(x) - T(c)\/T(c). A range of theoretical is sues thus raised is discussed, including other conceivable options (sp herical model, tricriticality, etc.). Attention is drawn to Nabutovski i's work and various scenarios are illustrated with the aid of schemat ic phase diagrams containing multicritical points that could, in princ iple, separate two distinct universality classes of electrolyte critic ality. The advantages of examining a basic four-state lattice model th at allows for ionic association-dissociation, etc., are reviewed. The issue of the existence, location, and nature of the long-heralded but still elusive gas-liquid transition and critical point in the continuu m restricted primitive model (hard spheres carrying charges +q and -q) is taken up in further detail. Earlier theoretical work and recent Mo nte Carlo simulations are summarized. In an effort to obtain a physica lly transparent, semiquantitative description, the work of Debye and H uckel and its subsequent elaboration via Bjerrum's concept of bound io n pairs is revisited and seen to predict phase separation and critical ity. Recent work by Levin and the author is described which repairs se rious defects of the earlier theories by including the interaction of the ion-pair dipoles with the screening ionic fluid, following Debye-H uckel methods. The resulting mean field theory agrees quite well with the simulations and appears to embody the most crucial physical effect s. However, the role of critical fluctuations, the related interplay o f the charge and density correlation functions, the likelihood of Isin g-like behavior, and the associated crossover scale t(x) remain import ant unsettled questions. An Appendix presents a critique of arguments by Stell to the effect that the restricted primitive model should disp lay Ising behavior and that 1/r4 effective interactions might be signi ficant.